Rush Casey E, Johnson Jared, Burroughs Samantha, Riesgaard Brandon, Torres Alejandro, Meunier-Goddik Lisbeth, Waite-Cusic Joy
Food Science and Technology, Oregon State University, Corvallis 97331.
JDS Commun. 2022 Jan 10;3(2):91-96. doi: 10.3168/jdsc.2021-0168. eCollection 2022 Mar.
Rework is a common practice used in the dairy industry as a strategy to help minimize waste from processing steps or errors that might otherwise render the product unsaleable. Dairy processors may rework their high-temperature, short-time (HTST) fluid milk products up to code date (21 d) at a typical dilution rate of ≤20% rework into ≥80% fresh raw milk. Bacterial spores present in raw milk that can survive pasteurization and grow at refrigeration temperatures are often responsible for milk spoilage. However, the potential impact of growth and thermal resistance of organisms in reworked product has not been investigated. Our objective was to characterize growth, sporulation, and thermal resistance of under conditions representative of extreme storage conditions (time and temperature) of reduced fat (2%) and chocolate milk to evaluate whether product containing rework would have a reduced shelf life. Commercial UHT-pasteurized 2% milk and chocolate milk were independently inoculated with 4 strains of at 1 to 2 log cfu/mL and stored at 4°C and 7°C for 30 d. Changes in cell densities were determined by standard serial dilution with spread plating on tryptic soy agar with yeast extract and incubation at 25°C for 48 h. Spore counts were determined following thermal treatment at 80°C for 12 min. Thermal resistance of a cocktail of in milk was determined after treatments at 63°C for 30 min and 72°C for 15 s. Strains of grew rapidly at 7°C and reached a maximum cell density of ~8 log cfu/g in both 2% and chocolate milk within 12 d. All strains grew more slowly at 4°C and had not reached maximum cell density by 21 d. With extreme temperature abuse (25°C, 24 h), will sporulate in milk; however, thermally resistant subpopulations, including spores, did not develop in milk at 4°C until after stationary phase was achieved (>24 d). Vegetative cells of were verified to be sensitive to pasteurization (>7 log reduction); therefore, would not be expected to contribute to reduced shelf life of fluid milk products containing rework, even with extended storage before rework.
返工是乳制品行业的一种常见做法，作为一种策略，有助于将加工步骤产生的废料或可能使产品无法销售的错误降至最低。乳制品加工商可以对其高温短时间（HTST）液态奶产品进行返工，直至保质期（21天），典型的稀释率是≤20%的返工产品与≥80%的新鲜原料奶混合。原料奶中存在的能在巴氏杀菌后存活并在冷藏温度下生长的细菌孢子通常是牛奶变质的原因。然而，返工产品中微生物的生长和耐热性的潜在影响尚未得到研究。我们的目标是在低脂（2%）牛奶和巧克力奶极端储存条件（时间和温度）的代表性条件下，对[具体微生物名称未给出]的生长、产孢和耐热性进行表征，以评估含有返工产品的保质期是否会缩短。将市售超高温瞬时灭菌的2%牛奶和巧克力奶分别接种4株[具体微生物名称未给出]，接种浓度为1至2 log cfu/mL，在4°C和7°C下储存30天。通过在含有酵母提取物的胰蛋白胨大豆琼脂上进行标准系列稀释和平板涂布，并在25°C下培养48小时来测定[具体微生物名称未给出]细胞密度的变化。在80°C热处理12分钟后测定孢子数。在63°C处理30分钟和72°C处理15秒后，测定牛奶中[具体微生物名称未给出]混合菌液的耐热性。[具体微生物名称未给出]菌株在7°C下生长迅速，在12天内，2%牛奶和巧克力奶中的细胞密度均达到约8 log cfu/g的最大值。所有菌株在4°C下生长较慢，到21天时未达到最大细胞密度。在极端温度滥用（25°C，24小时）条件下，[具体微生物名称未给出]会在牛奶中产孢；然而，耐热亚群，包括孢子，在4°C的牛奶中直到达到稳定期（>24天）后才会形成。已证实[具体微生物名称未给出]的营养细胞对巴氏杀菌敏感（>7个对数级减少）；因此，即使在返工前延长储存时间，[具体微生物名称未给出]也不太可能导致含有返工产品的液态奶产品保质期缩短。
